Many technologies rely on ultrathin dielectric layers to act as tunnel barriers between two electrodes to form metal-insulator-metal (MIM) structures. For example, magnetic tunnel junctions (MTJs), which have contributed to the rapid miniaturization of computer memories, are simply two metallic ferromagnetic thin film electrodes with a ˜0.1 nm-2 nm dielectric layer between them. The figure-of-merit tunnel magnetoresistance (TMR), defined as the ratio of the resistance of the device when the ferromagnetic layers are magnetized in parallel and anti-parallel directions, depends critically on the thickness of the dielectric layer. The TMR oscillates with the thickness of the dielectric layer with a period of only ˜0.3 nm, so subnanometer thickness control of ultrathin films is necessary. Another example is the Josephson junction (JJ), a superconductor-insulator-superconductor (SIS) device used in voltage standards, superconducting quantum interference devices (SQUIDs), and quantum bits (qubits). A leak-free tunnel barrier with thickness much smaller than the superconducting coherence length is typically required for the superconductor electrodes to remain phase coherent. Further, because the critical current through the JJ decays exponentially with increasing tunnel barrier thickness, in Nb—Al/AlOx/Nb JJs the AlOx tunnel barrier thickness is typically on the order of 1 nm. (See L. A. Abelson and G. L. Kerber, Proceedings of the IEEE 92 (10), 1517 (2004).)
Producing an ultrathin, uniform, and leak-free dielectric film is difficult on metal substrates due to the naturally formed native oxides on most metals such as Nb. Nb—Al/AlOx/Nb JJs are an excellent example. Techniques which have been used include thermal oxidation schemes, Molecular Beam Epitaxy (MBE) and Chemical Vapor Deposition (CVD). However, these techniques suffer from a number of drawbacks such as numerous defects in the dielectric film, excessive complexity/cost, insufficiently thin dielectric films, and/or dielectric films with non-uniform thicknesses.
Atomic Layer Deposition (ALD) is also a chemical process like CVD, but it differs from CVD in terms of its self-limiting growth mechanism, which allows thickness precision at the atomic scale. ALD produces atomic layer-by-layer growth via sequential exposure of relevant chemical sources following well-defined chemical reactions. Taking Al2O3 as an example, alternating pulses of H2O and trimethylaluminum (TMA) are exposed to heated substrates, separated by a flush of inert carrier gas to assure the two chemicals never meet in a gaseous state. Growth of Al2O3 occurs via ligand exchange between H2O and TMA at the sample surface and is described by the chemical reactionsAlOH*+Al(CH3)3→AlOAl(CH3)2*+CH4  (Equation 1)AlCH3*+H2O→AlOH*+CH4  (Equation 2)where an asterisk denotes a surface species. There are several unique merits associated with the ALD process. First, ALD is a relatively low temperature process with ALD Al2O3 typically occurring near 200° C. This low thermal budget is particularly important to monolithic devices on Si-readout circuits. Another merit is that the involved chemical reactions occur only on the sample's surface, and the reactions stop at the completion of each exposure. ALD growth is hence self-limiting. In each cycle of ALD Al2O3, i.e. after both the reactions shown in Equations (1) and (2) have occurred once, only one molecular layer is produced, or about 1.2 Å thickness of Al2O3. This provides atomic-scale control of film thickness. Finally, ALD coatings are highly conformal, which is particularly important to coating surfaces with large aspect ratios. A large variety of films, including metals and dielectrics, can be grown using ALD as long as the sources for the relevant chemical reactions are available.
The quality of ultrathin films depends critically on their nucleation on substrates (or “M” electrode in MIM structures), which means substrate surface preparation is a key towards achieving leak-free tunnel barriers using ALD. The chemical reactions in an ALD process (for example, ALD Al2O3 given in Eqns. 1-2) require the existence of surface species, particularly hydroxyl surface groups (OH*) or methyl surface groups (CH3*). This requirement is automatically satisfied on certain substrates, such as SiO2 since residual H2O on the surface produces a well hydroxylated surface ready for ALD nucleation. However, for substrates that are poorly hydroxylated, such as hydrogen terminated silicon (H—Si), nucleation is frustrated due to the lack of reaction sites on the surface. While the dangling hydrogen bonds on H—Si do serve as reaction sites to some degree, the initial stages of growth are dominated by the formation of a ˜1 nm thick silicate interfacial layer (IL). (See Martin M. Frank, Yves J. Chabal, and Glen D. Wilk, Applied Physics Letters 82 (26), 4758 (2003).) However, surface activation, such as pre-exposing the H—Si to a large dose of TMA for ALD Al2O3 growth, has been shown to reduce the IL to ˜0.5 nm for an Al2O3 film with a total thickness of ˜3 nm. (Id. and see Xu Min, Lu Hong-Liang, Ding Shi-Jin, Sun Liang, Zhang Wei, and Wang Li-Kang, Chinese Physics Letters 22 (9), 2418 (2005).)
Similarly to SiO2 and H—Si, metallic substrates can be classified into two categories; those with a reactive surface, such as Al and Cu, and those without, such as Au and Pt. In the former case, for ex situ deposited metals, a native oxide of several nanometers (up to ˜5 nm for Al) will pre-exist, and ALD growth occurs easily on top. (See M. D. Groner, J. W. Elam, F. H. Fabreguette, and S. M. George, Thin Solid Films 413, 186 (2002) and A. J. Elliot, G. Malek, L. Wille, R. T. Lu, S. Han, J. Z. Wu, J. Talvacchio, and R. Lewis, Applied Superconductivity Conference 2012 (2012).) For in situ deposited metals, an IL may form from thermal oxidation or chemisorption of the ALD precursors, and this IL may range in thickness from ˜0.4 nm on in situ ALD-W (R. K. Grubbs, C. E. Nelson, N. J. Steinmetz, and S. M. George, Thin Solid Films 467, 16 (2004)) to ˜2 nm on in situ sputtered Al (A. J. Elliot, G. Malek, L. Wille, R. T. Lu, S. Han, J. Z. Wu, J. Talvacchio, and R. Lewis, Applied Superconductivity Conference 2012 (2012)). On noble metals, such as Pt, Ir, and Ru, nucleation of ALD films can be completely frustrated during the first 30-50 cycles of growth. (See K. Kukli, M. Ritala, T. Pilvi, T. Aaltonen, J. Aarik, M. Lautala, and M. Leskelä, Materials Science and Engineering B 118, 112 (2005).) These initial cycles act as an incubation process to prepare the surface for nucleation by adsorbing source material on the surface, effectively increasing its reactivity. ILs several nm thick have been reported when growing ALD dielectric films on noble metals, and they form through the diffusion of source material into the metal film, such as the diffusion of Tetrakis(ethylmethylamido)hafnium(IV) (TEMAH) into Pt during the growth of HfO2. (See K. Kukli, T. Aaltonen, J. Aarik, J. Lu, M. Ritala, S. Ferrari, A. Hårsta, and M. Leskelä, Journal of the Electrochemical Society 152, F75 (2005) and C. Chang, Y. Chiou, C. Hsu, and T. Wu, Electrochemical Solid State Letters 10 (3), G5 (2007)). The exact thickness and composition of the IL depend on the substrates and sources used. But, in the case of ALD-HfO2 on Pt, the IL thickness can be reduced from ˜10 nm to ˜5 nm and the interface can be made more uniform by exposing the metal film to a hydrous plasma to promote surface oxidation before ALD dielectric layer growth. (See C. Chang, Y. Chiou, C. Hsu, and T. Wu, Electrochemical Solid State Letters 10 (3), G5 (2007).) In either case of reactive or noble metals, the IL issue must be addressed in order to produce an ultrathin dielectric tunnel barrier using ALD on a metal substrate with minimized IL effect for tunnel junctions and many other MIM structures.